'Appendix C: Convenient Formulas for Gradient Operations' (Electromagnetic Waves, 2011), 'Introduction to Adaptive Arrays', 2nd ed., Chap. appendixc, pp. 515-516, DOI: 10.1049/SBEW046E_appendixc
IET Digital Library, https://digital-library.theiet.org/;jsessionid=5b6tmkigatvu8.x-iet-live-01content/books/10.1049/sbew046e_appendixc